The present invention relates to an improvement in drilling apparatus to substantially reduce the incidence of drill string breakage occasioned by abrupt deceleration of the drill string following a failure in the drill string support apparatus.
One prior art type of drilling apparatus utilizes a power swivel which is guided along spaced, parallel, substantially vertical guide rails, such that the power swivel is movable vertically. The power swivel includes means for supporting a string of down-hole tubulars such as drill pipe, and the drilling apparatus includes means for raising and lowering the power swivel so as to control the position of the string.
One important problem previously encountered with such drilling apparatus relates to damage to the string caused by a failure of the swivel positioning means. For example, in the event the cables or other structures used to support, raise and lower the swivel fail, the swivel and the attached string can drop vertically. Typically, if the swivel is dropped it decelerates abruptly when it strikes the platform on which the guide rails are mounted. Such abrupt deceleration can result in breakage of the string in the borehole, which represents a severe setback to drilling operations. In addition, a falling swivel can severely damage the platform and the guide rails of the drilling apparatus.